The present invention relates to nuclear magnetic resonance, and more particularly, to an apparartus and method for nuclear magnetic resonance and analysis of specimens remote from the source of a homogeneous polarizing magnetic field.
The phenomenon of nuclear magnetic resonance was discovered about 1948. Soon thereafter it became apparent that nuclear magnetic resonance has applications in connection with well-logging since with NMR techniques it is possible to derive a signal from fluids in rock pores and not from the rock itself. Previously known nuclear magnetic logging apparatus have employed the approach characterized by tipping magnetic nuclei, such as hydrogen nuclei in oil or water, away from their original alignment with the earth's magnetic field by application of a strong dc magnetic field produced by passing a current through a coil in the well-logging tool. This magnetic field was then reduced to zero as rapidly as possible. The hydrogen nuclei then rotates about the earth's magnetic field at a characteristic frequency of about 2 kHz in the one-half gauss magnetic field strength of the earth. These rotating nuclear magnets induce a 2 kHz voltage in a receiver coil included in the well-logging tool. Due to the relatively low strength of the earth's magnetic field, the signal-to noise ratio is extremely low.
Other practical problems are also apparent. For example the dc current in the coil employed to produce a magnetic fringe field to rotate the hydrogen nuclei in the water or hydrocarbons cannot be reduced to zero in less than about 20 milliseconds. This 20 millisecond period cannot be used for measuring purposes and is termed "recovery time". A recovery time this long results in much important information from the precessing nuclei being lost. Since the signal-to-noise ratio varies approximately as the strength of the magnetic field to the three halves power, sensitivity decreases rapidly with decreasing field. Therefore it is apparent that the weak earth's field results in extremely low signal-to-noise ratio. Furthermore, this approach does not allow measurement of important relaxation times associated with nuclear resonance. Knowledge of these relaxation times, to achieve certain states of equilibrium following a disturbance, is essential for extracting information about molecular diffusion of the molecular environment.
The most severe practical drawback in previously known nuclear magnetic logging apparatus is the overwhelming signal from the drilling mud in the borehole. This signal, from water protons, almost completely masks any signal of interest from the formation. This problem was solved heretofore by the procedure of adding chemicals to the borehole fluid to suppress the unwanted signal. Since this procedure can take many hours, no other logs can be run and of course no drilling can take place. As best, previously known nuclear magnetic logging tools were limited in range to the area of the formation within about an inch from the borehole.
Other nuclear magnetic resonance techniques have been employed in the laboratory. A large uniform magnetic field is obtained at the sample by placing the sample between the poles of a large magnet. The nuclei will rotate (or "precess") about the applied field at a characteristic frequency (called the "Larmor" frequency). For hydrogen nuclei (protons) the characteristic frequency is 4.25 kHz per Gauss of applied field. Thus, in a typical laboratory magnet field of 10,000 Gauss the frequency is .about. 42.5 MHz. If a radio frequency magnetic field is applied perpendicular to the polarizing dc field, and if the frequency of the rf field is adjusted to the same frequency as the rotating nuclei (adjusted to be in "resonance"), some of the muclei can accept energy from the rf field and be "flipped" so as to be aligned against the field direction. An rf field at the sample is obtained by means of an rf coil surrounding the sample. Either a continuous rf field or rf pulses may be employed. Of particular interest is the employment of short pulses of rf power at the characteristic Larmor frequency. The observation of the nuclear spin system is made after the rf is turned off, when the voltage induced in a receiver coil by the precessing nuclei is recorded and analyzed.
Pulse methods are often much more efficient than those obtained by sweeping the rf frequency. Relaxation plays an important role in pulse experiments, and as a result pulse techniques provide the most generally useful method of measuring relaxation times. Pulse methods permit measurement of relaxation times for individual NMR lines and complex molecules. Sophisticated pulse techniques have been developed for NMR chemical analysis applications. Such pulse methods are discussed in "Pulse and Fourier Transform NMR: Introduction to Theory and Methods," Thomas C. Ferrar and Edwin D. Becker, published 1971 by Academic Press.